This invention relates to an improvement in the use of the known type of oxygen sensor comprising a solid oxygen-ion-conducting electrolyte with porous, thin layer or film, metal (e.g. platinum) electrodes attached on substantially opposite surfaces of the electrolyte. When each electrode of this type of sensor is in contact with a different oxygen concentration and the electrodes are connected in an electrical measuring circuit, oxygen ions migrate through the electrolyte between the electrodes coincidently with a flow of electrons in the circuit generating a measurable voltage or electromotive force between (or across) the electrodes or two points in the circuit.
This type of sensor has been known for use in monitoring: (1) exhaust gases of internal combustion engines in thermodynamic nonequilibrium for control of the air-fuel ratio in the combustion process, (2) stack or flue gases of industrial combustion furnaces for control of the combustion process to eliminate smoke and other undesirable emissions, and (3) furnace atmospheres of metal heat treating and other furnaces in substantial thermodynamic equilibrium for control of their oxygen potential, e.g. in nonoxidizing and reducing gas atmospheres.
However, durability of the thin layer or film platinum electrode in contact with the monitored flowing hot gases has been a problem. Such electrode has been variously noted to be adversely affected by thermal shock and differential expansion stresses in the sensor, mechanical abrasion and impact stresses caused by particles carried in the flowing gases, and chemical reaction effects with constituents in the gases being monitored.
The presence of metal vapors in nonoxidizing or reducing atmospheres of metal heat treating furnaces have been noted in U.S. Pat. No. 3,645,875 to cause alloying with and embrittlement of the platinum film electrode which does not serve any catalytic function in this environment. In conjunction with the reducing conditions at high temperatures, the bond (and electrical continuity) between such electrode and the solid electrolyte is also noted to be adversely affected. Of course, it has been known for some time (see Metals Handbook, 8th Edition, Vol. 1, pp. 1178 and 1190, published 1961 by the American Society for Metals) that platinum in hot reducing atmospheres is attacked and contaminated by phosphorus, arsenic, lead, bismuth, antimony, silicon, iron and manganese. Moreover, in our recent studies of platinum film electrodes tested in contact with operating atmospheres of steel carburizing furnaces, we discovered the principal platinum alloying contaminant to be zinc, although other contaminants found included iron, titanium, nickel, boron, lead, chromium, sodium and copper.
The remedy suggested by U.S. Pat. No. 3,645,875 for the problem of platinum contaminants in reducing atmospheres is to adherently attach a porous, thin, protective overlayer on the platinum film electrode, which overlayer is formed of refractory material such as zirconia. A similar remedy for mechanical and chemical damages to platinum film electrodes exposed to automotive exhaust gases is proposed in U.S. Pat. No. 3,978,006. However, U.S. Pat. No. 4,164,462 notes that such overlayers can suffer a durability problem (cracking) of their own. The latter fact was confirmed in our own studies, which showed that such overlayers (e.g. of alumina cement) readily crack and spall off, thereby leaving significant portions of the platinum film electrode unprotected. The further consequence of the foregoing results was embrittled (mainly by zinc alloying) and cracked platinum film electrodes such that those electrodes could easily be removed from the electrolyte by scraping with a finger nail. Additionally, trying to improve the strength and adherence of the overlayer by firing it at higher temperatures above about 1150.degree. C. is often unsatisfactory, especially for sensors with stabilized zirconia electrolytes to be used in metal heat treating processes. Besides the possibility of destroying the needed porosity by sintering the overlayer too dense, reheating of the stabilized zirconia electrolyte (while firing the overlayer) above about 1150.degree. C. causes a change in the zirconia structure, which in turn causes the electrolyte to exhibit sluggish nonideal behavior in service at temperatures below about 1150.degree. C. as is often the case in metal heat treating processes.
With respect to monitoring internal combustion engine exhaust gases with sensors of the type described above, U.S. Pat. No. 4,021,326 notes the problem of catalyst poisons which interfere with the catalytic activity of the catalytic (e.g. platinum) film electrode. Such activity is needed to catalyze the formation of thermodynamically stable gases (i.e. gases in thermodynamic equilibrium) from the nonequilibrium exhaust gases prior to contacting the electrolyte with such stable gases. The remedy suggested by this patent for such poison problem is the application of a getter for the catalytic poisons in or on top of the thin, porous, protective layer covering the catalytic film electrode. Identified poisons are heavy metals such as lead, copper, zinc, and nonmetals such as sulfur and halogen compounds. Suitable getters for these poisons are noted to be gold, silver, nickel and/or nickel oxide and silicon dioxide. It is to be noted, however, that catalyst poisons are not conceptually nor quantitatively the same as embrittling alloy contaminants of an electrode as noted in U.S. Pat. No. 3,645,875.
Further improvements in oxygen sensor devices for monitoring internal combustion engine exhaust gases have been revealed for supplementing the capacity of the catalytic (platinum) film electrode to promote the needed thermodynamic equilibrium in those gases for more accurate sensor operation. U.S. Pat. Nos. 3,935,089 and 4,097,353 suggest incorporating platinum into the thin-layer, porous, protective coating on the film electrode that is contacted by the monitored gases. U.S. Pat. No. 4,132,615 discloses passing the nonequilibrium exhaust gases through a catalyst mass (such as a bed of alumina pellets containing platinum catalyst) before contacting the oxygen sensor. U.S. Pat. No. 4,140,611 teaches the use of a honeycomb type oxidation catalyzer (otherwise known to contain platinum catalyst) in the same manner as the catalyst mass of the preceding patent.
U.S. Pat. No. 4,121,989 shows a specially tailored oxygen sensor device for greater efficiency, accuracy and reproducible operation in monitoring stack gases from industrial combustion furnaces to control the degree of combustion and combustion efficiency with excess air. Such device has felted ceramic fiber discs partly embedded into fired, thin-layer, paste-derived, platinum electrodes and thermally reduced, high surface area, platinum particles dispersed over the electrode surfaces and within the felted discs as a result of applying chloroplatinic acid through the felted discs to the electrode surfaces. Such particles augment the capacity of the platinum electrodes to effect the ionization-deionization reactions of oxygen in the device.